A known method for increasing the toughness of a ceramic body is to incorporate weak interphases or interfaces into the material. As an example, it is common to reinforce a ceramic matrix with continuous fibers which are coated with a thin layer of either boron nitride or carbon. The carbon or boron nitride creates a weak interface between the matrix and the fiber which allows sliding between the fiber and the matrix and/or causes propagating cracks to be deflected along the interface or within the interphase. These events allow the reinforcing fibers to remain intact and continue to reinforce the ceramic body and to arrest additional crack propagation. Similar interfacial failures can occur in particulate and whisker reinforced materials which contain weak interfaces. In these instances microcracking and crack deflection characteristics result in toughening of the ceramic body. In multilayered laminar composites, multiple parallel interfaces may fracture, thus increasing the work of fracture of the entire ceramic body.
Typical approaches to creating a weak interface between reinforcing phases and matrix phases incorporate interphase compounds which are characterized by layered crystal structures containing a crystal plane exhibiting weak shear properties. This characteristic promotes interfacial debonding and fiber pullout toughening (i.e., fiber frictional sliding) mechanisms. Materials commonly used include carbon, boron nitride, micaceous materials such as fluorphlogophite such as disclosed by U.S. Pat. No. 4,935,387 to Beall et al., and beta-alumina/magnetoplumbite compounds such as disclosed by U.S. Pat. No. 5,137,852 to Morgan et al.
Since many ceramic composites are used in high-temperature oxidizing environments (typically at temperatures exceeding 1100.degree. C.), the reinforcement/matrix interphase must be oxidation resistant and stable thermodynamically with both the reinforcement and the matrix phase. Carbon and boron nitride materials oxidize readily and are unsuitable for use in composites requiring long-term service at temperatures greater than 600.degree. C. In addition, boron nitride is rapidly degraded by water vapor making its use as an interphase material in combustion atmospheres, and other water vapor laden environments, very limited. Fluorphlogophite and beta-alumina/magnetoplumbite compounds have a tendency to react with many of the continuous fiber reinforcements currently commercially available.
Some non-layered oxide compounds have been studied as interphase in continuous fiber ceramic composites. These include tin oxide, zirconium oxide, and zirconium tin titanate. These compounds were considered for alumina reinforced composites because of the lack of reactivity between these interphase compounds and alumina. It was determined that these compounds work well as reaction barriers between matrix and reinforcement, but they do not provide the weak interphase or interface needed to cause crack deflection and fiber pullout toughening.
Monazite and xenotime compounds, phosphates with the general formula APO.sub.4, in which A represents trivalent rare earth elements of the lanthanide series, are non-layered crystal compounds which are disclosed for creating weak interfaces in ceramic composites by U.S. Pat. No. 5,514,474 to Morgan et al. These materials appear to provide a weak interface, rather than a weak interphase, by exhibiting high interfacial energies (low bonding) with possible reinforcements and matrices. Although some promising preliminary results have been demonstrated with monazite and xenotime in model composite systems, incorporation of these interfaces into actual continuous fiber reinforced ceramic composites has yielded mixed results.